Guozhen Liu,Yanan Guo,Baochun Meng,Zhenggang Wang,Gongping Liu,Wanqin Jin

State Key Laboratory of Materials-Oriented Chemical Engineering,College of Chemical Engineering,Nanjing Tech University,Nanjing 211816,China

Keywords:Two-dimensional material MXene membranes Hollow fiber Desalination Nanofiltration

ABSTRACT Two-dimensional material membranes with fast transport channels and versatile chemical functionality are promising for molecular separation.Herein,for the first time,we reported design and engineering of two-dimensional Ti3C2Tx MXene(called transition metal carbides and nitrides)membranes supported on asymmetric polymeric hollow fiber substrate for water desalination.The membrane morphology,physicochemical properties and ions exclusion performance were systematically investigated.The results demonstrated that surface hydrophilicity and electrostatic repulsion and size sieving effect of interlayer channels synergistically endowed the MXene hollow fiber membrane with fast water permeation and efficient rejection of divalent ions during nanofiltration process.

1.Introduction

Nowadays,the shortage of fresh water is still a global problem with continuing economic development and population increasing[1].Seawater desalination is considered as an effective strategy to solve this crisis.Among all the developed technologies for desalination,membrane process with low energy consumption and high efficiency has becoming a promising technology to obtain fresh water from seawater [2,3].Traditional membrane materials were dominated by the polymer,however,which generally suffer from the trade-off relationship between the permeability and selectivity[4].It is essential to develop novel membrane materials for overcoming this trade-off effect.Recently,featuring with unique physicochemical properties and atomic thickness as well as welldefined nanochannels,2D materials have aroused tremendous research interests [5,6].Besides,2D materials such as graphene oxide (GO),metal organic frameworks (MOFs) nanosheets,and MoS2have also been assembled into synthetic membranes towards small molecules sieving,achieving outstanding separation performance [7–13].Nevertheless,membranes based on 2D materials such as GO are easily swelling in water to cause the stability issue[14,15].Although many attempts have been made to restrain the membrane swelling in water via chemical or physical modification[16–19],novel 2D materials with structural robust are still needed to be developed.

Recently,a new member of 2D material family called metal carbides and nitrides(labeled as MXene)has emerged.MXene have a formula with Mn+1XnTx(n=1,2,or 3),where M stands for an early transition metal,X is carbon and/or nitrogen.In addition,MXene powders were often made by selective etching of element A from its precursor Mn+1AXn(A is an IIIA or IVA element) in strong acid[20].After sonication of the powders in water solution,MXene nanosheets can be obtained.It should be noted that the wetetching process endows MXene nanosheets terminated with abundant functional groups Txincluding-OH,-O and-F,which favors MXene with well hydrophilicity,versatile chemical tunability and negative charged[21,22].Meanwhile,owing to strong van der Wall force or hydrogen bonding,MXene could overcome the water swelling to maintain structural integrity[14,23].Therefore,MXene nanosheets can be as building blocks for constructing laminar membranes with 2D transport nanochannels for water treatment applications including desalination,dye rejection or organic solvents dehydration [24–28].

Until now,there are only a few reports on MXene-based membranes for water desalination.Wang and co-workers[16,29]fabricated the Al3+intercalation and self-crosslinked MXene membrane on flat nylon substrate for desalination during forward osmosis process.Outstanding desalination performance with high salts rejection was achieved owing to remarkable improvement of membrane anti-swelling property,while the low pressure driven nanofiltration process remains to be demonstrated.Very recently,our group reported the fabrication of surface-charged MXene membranes by coating a polyelectrolyte layer for nanofiltration desalination,achieving efficient rejection for divalent salts [30].Furthermore,most reported MXene separation membranes were freestanding or supported on small-area discs,such as anodic aluminum oxide(AAO),polyacrylonitrile(PAN)or polyvinylidene fluoride (PVDF) [22,31–33].They usually suffer from the weak interface adhesive strength and difficulty on scale-up,which may hinder their practical application.Compared with flat-sheet substrates,hollow fiber can achieve higher packing density in membrane module [34–36].Recently,MXene membrane are also deposited on inorganic hollow fiber for molecular separation.For example,Cui et al.[37]designed and fabricated TiO2-MXene membranes on the Al2O3hollow fiber via dip-coating,which exhibited a high and reproducible filtration performance with water flux of＞900 L·m-2·h-1·MPa-1and a cut-off molecular weight of～22,000 Da.Jing et al.[38] also added MXene nanosheets into TiO2hydrosol to make homogenous coating solution and then deposited it on four-channel Al2O3hollow fiber support.The obtained MXene-TiO2layer displayed ideal pathway(longitudinal-lateral transport nanochannel)for dextran molecules between TiO2nanoparticles and MXene nanosheets,showing a cut-off molecular weight of 14,854 Da and a high pure water flux of 1020 L·m-2·h-1·MPa-1.Besides,Xu et al.[39] also developed self-crosslinked MXene membranes on yttria-stabilized zirconia(YSZ)hollow fibers for H2separation.The as-prepared MXene hollow fiber membranes with controllable interlayer space displayed an outstanding gas separation performance with good operational stability over a continuous operation of 120 h.Meng et al.[40]also intercalated Ni2+into MXene nanosheets to assemble laminar MXene membrane on Al2O3hollow fiber via vacuum filtration.Owing to the strong interaction between the negatively charged MXene nanosheets and Ni2+,the interlayer space was finely tuned.The resulted MXene membrane showed excellent H2/CO2mixture separation performance with H2permeance of 8.35 × 10-8mol·m-2·s-1·Pa-1and separation factor of 615.However,compared with inorganic hollow fiber,polymeric one exhibits lower cost and easier processing.To the best of our knowledge,MXene membranes supported on organic hollow fiber are not explored yet.

Herein,for the first time,we proposed and engineered MXene laminar membranes supported on polymeric hollow fiber for ions exclusion from water(Fig.1).Atomic-thick MXene nanosheets were synthesized via a wet-etching process and then stacked on the outer surface of PAN hollow fiber to construct laminar membranes via a vacuum suction method.The membrane microstructures,physicochemical properties and ions exclusion performance were systematically investigated.

Fig.1.Schematic of MXene/PAN hollow fiber composite membrane for ions exclusion from water.

2.Experimental

2.1.Materials

The MAX powders (Ti3AlC2) with particle size of～74 μm were purchased from 11 technology Co.,Ltd China.LiF (99.9%) powders were provided by Aladdin Industrial Corporation,Shanghai,China.HCl (36%–38%) solution was obtained from Lingfeng Chemical Reagent,China.Besides,Ethanol (＞99.7%) were supplied by Yasheng Chemical Co.Ltd.PAN hollow fiber substrate with surface pore size of several tens of nanometers,external diameter of 2 mm and inner diameter of 1.3 mm was supplied from Nanjing Junxing Jinhui Film Environmental Protection S&T.Deionized water was used in the all experiments.

2.2.Synthesis of Ti3C2Tx MXene nanosheets

The method to synthesize Ti3C2TxMXene nanosheets followed our previous works [41,42].Typically,LiF (0.67 g) were dissolved in 10 ml of HCl solution (6 mol·L-1).Then Ti3AlC2powders (1 g)were added into the above solution slowly,following by stirring for 24 h under 35 °C.The resulting product was washed using water and ethanol for several times until the pH of the supernatant was closed to 7.The sediment was re-dispersed into DI water with ultra-sonication for 60 min under the Nitrogen atmosphere.Finally,well-exfoliated MXene solution with the concentration of～0.35 mg·ml-1could be obtained after centrifugation at 3500 r·min-1for 60 min.

2.3.Fabrication of MXene/PAN hollow fiber membranes

The well-delaminated Ti3C2TxMXene nanosheets were stacked on the outer surface of PAN hollow fiber via a facile vacuum filtration.Specifically,one side of the PAN hollow fiber was sealed using glue and the other side was connected to a vacuum pump.Meanwhile,a certain volume of MXene solution were diluted into 250 ml DI water via magnetic stirring.The resulting MXene nanosheets could be easily deposited on the surface to form laminate separation layer with pressure driving.The membrane thickness could be finely regulating by changing the filtration time or the concentration of MXene solution.

2.4.Membrane performance evaluation

The membrane separation measurement was conducted using a home-made filtration device with dead-end mode under 1 bar at room temperature.The effective membrane area for testing is about 3 cm2.The feed concentration of around 50 mg·L-1is often used to simply evaluate nanofiltration performance of 2Dmaterial membranes including graphene oxide membranes and MXene membranes [43,44].To keep consistent with literature and make a fair comparison,we chose this salt concentration.It is also noticed that higher salt concentration would decrease the salt rejection due to the weaken electrostatic effect of membrane surface.The separation performance was evaluated with salts MgCl2,NaCl,Na2SO4and MgSO4.Each data was obtained after one hour when the filtration system reached stable state.The water permeance(J)and salts rejection(R)were calculated according to the following equations:

where V (L) is the volume of the permeate side.A (m2) responds to the effective membrane area.P (MPa) refers to the operation pressure and t (h) is the permeation time,respectively.Besides,Cpand Cfare the salts concentrations of permeate and feed sides,respectively.

2.5.Characterization

X-ray diffraction (XRD) patterns were recorded on a Rigaku Miniflex 600 diffractometer.Field emission scanning electron microscopy (FESEM,JSM-7600F,Japan) and Atomic Force Microscope (AFM,XE-100,Park SYSTEMS,Korea) were applied to study the morphological characteristics of the materials.The physicochemical properties of as-synthesized samples were analyzed by Fourier Transform Infrared Spectrometer (FTIR,AVATAR-FT-IR-360,Thermo Nicolet,USA),X-ray photoelectron spectroscopy(XPS,Thermo ESCALAB 250,USA),the contact angle measurement system (DSA100,Kruss) and streaming potential measurements with an electrokinetic analyser (SurPASS3,Anton Paar,Austria).The salts concentrations in feed and permeation sides were obtained using electrical conductivity (FE38-Standard,METTLER TOLEDO,Switzerland).

3.Results and Discussion

3.1.Characterization of MXene nanosheets

Ti3C2TxMXene was chosen as membrane materials due to its well hydrophilicity and easy synthesis [22,33].Ti3C2Txnanosheets were obtained from the precursor Ti3AlC2MAX powders using HCl and LiF as etching agents.As shown in Fig.2a,the MAX powders with tightly stacking structure shows a grey color;after etching the Al atom,the MXene powders in black color exhibit layered accordion-like structure (Fig.2b).Besides,the characteristic peak responding to MXene located at 2θ value of～7° was observed in XRD patterns (Fig.2c),indicating a successful etching process[45].The resulting MXene powders were dispersed in DI water via sonication to form homogeneous solution with abundant ultrathin nanosheets.Tyndall scattering effect was observed in the MXene solution shown in Fig.2d.The AFM image in Fig.2e displays MXene nanosheets with a thickness of～1 nm,which is accordance with the reported single layer MXene nanosheet [46].Besides,as shown in Fig.2f,TEM characterization confirmed the flexible of nanosheets with high crystallinity.These nanosheets could be assembled into laminar membranes with abundant channels shown in Fig.2g,which can be applied for selective molecular transport based on size sieving effect.In addition,the freestanding MXene laminar membrane with thickness of～8 μm can be bended by a tweezer without any breakage,demonstrating well mechanically robust (Fig.2g).

3.2.Morphology of MXene/PAN hollow fiber membranes

Owing to high packing density,easy scale up and costeffectiveness,the polymeric hollow fiber was employed as the substrate to support the laminar MXene separation layer.It exhibits sandwich like structure with pore size of tens of nanometers(Fig.3a-c),water and salts could pass through unimpeded.Here,a facile vacuum suction method was used to construct the MXene hollow fiber composite membrane.MXene nanosheets can be easily deposited on the outer surface of hollow fiber with driven by the pressure.Meanwhile,the membrane thickness can be controlled by altering the volume or the concentration of MXene solution.As shown in Fig.3d,compared with the blank substrate,the MXene membranes without visible pinholes or cracks exhibits black color,indicating the uniform distribution of nanosheets on the outer surface.It was verified by the surface SEM images in Fig.3e that the MXene nanosheets covered the pores of substrate(Fig.3a) evenly to form a typical 2D-material membrane surface morphology with wrinkles and wave-like ripples.Besides,it can be seen from the AFM image that MXene membranes surface also exhibits crumpled and wrinkled structure(Fig.3g and h),which is consistent with the morphologies of membranes in SEM images.Meanwhile,the membranes surface roughness is～21.6 nm(Fig.3g).The cross-section SEM image in Fig.3F and EDX mapping(element:Ti,C,O,F)in Fig.3i displays that a laminar MXene layer with thickness of～250 nm is firmed adhered onto the hollow fiber substrate.

Fig.2.Synthesis of MXene nanosheets.SEM images of (a) Ti3AlC2 MAX and (b) Ti3C2Tx MXene powders,the inserted are the digital photos of the powders,respectively.(c)XRD patterns of MAX and MXene powders.(d) MXene solution with concentration of～0.01 mg·ml-1,showing Tyndall effect in a colloidal suspension.(e) AFM image of MXene nanosheets with the thickness of～1 nm.(f)TEM image of MXene nanosheets with selective area electron diffraction pattern(SAED)pattern.(g)Freestanding MXene membrane with thickness of～8 μm before and after bending.

Fig.3.Morphology of MXene hollow fiber membrane.SEM images of(a)surface and(b)cross-section of the blank PAN hollow fiber;(c)The enlarged version with the outer dense layer (inserted) in (b);(d) Photographs of blank PAN hollow fiber (white) and MXene/PAN hollow fiber membranes (black).(e) Surface,(f) cross-section and (g) AFM image of MXene/PAN hollow fiber membranes.(h) 3D images corresponding to (g).(i) The EDX mapping of MXene membrane in (f).

3.3.Physicochemical properties of MXene/PAN hollow fiber membranes

We further studied the physicochemical properties of the resulting MXene membranes.The interlayer distance was analyzed by XRD characterization.As shown in Fig.4a,the peak belonging to MXene appeared at 2θ of～6.5°.Based on the Bragg equation,the dspacing of MXene membranes was calculated as～1.33 nm.The thickness of mono-layer MXene nanosheets was reported as～1 nm [46],hence,the interlayer distance of MXene membranes was～0.33 nm,which is between the diameter of water and various ions [47].Probably it can be served as separator to acquire fresh water from salts solution.Furthermore,the MXene membrane maintained negatively charge at pH ranging from～3 to 10(Fig.4b),which is expected to repel co-ions according to Donnan exclusion theory [19,48].Meanwhile,the water contact angle on the surface of MXene membrane was～48.9° shown in Fig.4c,demonstrating a hydrophilic membrane surface that could facilitate the transport of water molecules.

The chemical properties of MXene membranes were analyzed by IR and XPS.As shown in Fig.5a,there are two characteristic bands located at～3444 cm-1and～1635 cm-1,referring to -OH and C＝O,respectively.In addition,XPS characterization was used to investigate the functional groups.It can be seen from Fig.5b that Ti,C,O and F elements were detected,which is accord with the EDX in Fig.3f.High-resolution XPS spectra are shown in Fig.5c-f.There are five main peaks corresponding to Ti-C (454.3 eV),Ti (II) (455.1 eV),Ti (III) (455.9 eV),Ti-O (456.8 eV) and Ti-F(458.2 eV) in the Ti2p spectrum,respectively (Fig.5c).Besides,the O1s spectrum shown in Fig.5d could be fitted with four components located at 529.3 eV,530.3 eV,531.5 eV and 532.7 eV,which corresponding to O-Ti,O-Ti/OH,O-C/OH and H2O,respectively.Also,as shown in Fig.5e,four peaks in the C1s spectrum were observed,relating to C-Ti (281.5 eV),C-C (284.6 eV),C-O(286.0 eV) and C＝O (288.5 eV),respectively.Fig.5f shows the F1s spectra,which is composed of F-Ti (684.4 eV) and F-C(686.0 eV).The IR and XPS results confirm the chemical structures of MXene membrane,which agree with the reported literatures[33,49].

Fig.4.Physical properties of MXene hollow fiber membranes.(a) XRD patterns,(b) Zeta potential and (c) contact angle.

Fig.5.Chemical properties of MXene/PAN hollow fiber membranes.(a) IR spectra.(b) XPS scan and (c)-(f) Ti2p,O1s,C1s and F1s spectra,respectively.

3.4.Membrane separation performance

Nanofiltration with low energy consumption is considered as an alternative process to the separation of inorganic salts and small organic molecules similar to reverse osmosis,especially for divalent ions exclusion from water.Here,we evaluated the separation performance of MXene hollow fiber membranes for nanofiltration application.First,the effect of MXene membrane on the nanofiltration performance of Na2SO4solution was optimized.The thickness can be facilely adjusted via controlling the volume of MXene solution deposited on the substrate.As shown in Fig.6a,with increasing the membrane thickness,the water permeance continue to reduce due to the enhanced mass transport resistance.Owing to the pore size of hollow fiber is several tens of nanometers shown in Fig.3a,water and ions could pass through unimpeded.Therefore,the molecular sieving ability mainly depends on the interlayer distance of deposited MXene nanosheets.Interestingly,as verified by XRD in Fig.4a,the interlayer distance of MXene membranes is～0.33 nm,which is between the kinetic diameter of water(0.28 nm) and hydrated Na+(0.716 nm),where water molecules could penetrate through laminar MXene membrane while hydrated Na+was repelled.Thus,fast water transport and efficient rejection for ions can be achieved.Besides,based on the size sieving effect,to further improve the water permeance,the interlayer distance of MXene nanosheets should be as large as possible in the range within the kinetic diameter of water to that of hydrated ions.Some efficient strategies such as introducing nanoparticles and chemical modification can be employed to finely regulate the interlayer distance of MXene nanosheets.Furthermore,the membrane thickness can be decreased to further enhance the water permeance.Nevertheless,the membranes with thickness less than～90 nm exhibited low salt rejection,which may be determined by the easy-existence of non-selective defects when the MXene nanosheets were deposited on the PAN hollow fiber substrate with hook face.Surprisingly,further increasing the loading of MXene nanosheets,the defects could be efficient repaired,resulting in remarkable improvement and then remaining stable in salt rejection.Therefore,～90 nm was considered as the optimal membranes thickness for fabricating the MXene hollo fiber membrane.The resulting MXene hollow fiber demonstrated nanofiltration desalination performance with water permeance of～59 L·m-2·h-1·MPa-1and Na2SO4rejection of～70%.

Fig.6.(a)Effect of MXene/PAN hollow fiber membranes thickness on the separation performance for rejecting Na2SO4.(b)Nanofiltration performance of MXene/PAN hollow fiber membranes for various salts.

Fig.7.Stability of MXene hollow fiber membrane during continuous nanofiltration desalination process.

We further investigated the nanofiltration performance of MXene hollow fiber membrane for desalination of different salts solution (MgSO4,MgCl2and NaCl).As shown in Fig.6b,the salts rejection sequence of MXene membranes with negatively charges as characterized by zeta potential (Fig.4b) follows the order of ratio of charge number for the cation and anion,R(Na2SO4) ＞R(MgSO4) ＞R(MgCl2).According to the Donnan exclusion theory,the charged membrane surface repels co-ions with rejecting counter-ions in the feed solution to maintain electrical neutrality[19].Compared with MgSO4,lower rejection for NaCl was observed,demonstrating that the size sieving effect also contributed to the nanofiltration performance.

The membrane stability is essential for practical application.We further studied the stability of MXene hollow fiber membrane for desalination of Na2SO4solution during continuous nanofiltration process.Promisingly,as shown in Fig.7,high and stable separation performance was achieved during the continuous operating time up to 600 min,indicating that our developed MXene hollow fiber membrane is robust to against continuous operation without structural damage.

4.Conclusions

In summary,a novel MXene hollow fiber membrane was successfully fabricated by depositing MXene nanosheets on top of porous polymeric hollow fiber substrate via a vacuum suction method.The wet-etching process to synthesize MXene nanosheets with versatile oxygen containing groups endowed the MXene membrane with hydrophilicity and negatively charges.Meanwhile,interlayer channels were formed by stacking the MXene nanosheets.Owing to the synergic effect of water affinity,electrostatic repulsion and size sieving,the resulting MXene hollow fiber with membrane thickness of～90 nm exhibited water permeance of～59 L·m-2·h-1·MPa-1and Na2SO4rejection of～70%.Hollow fiber has unique advantage of higher packing density in membrane module,showing great potential in scale-up of MXene and other 2D-material membranes for water desalination.This preliminary work demonstrates the feasibility of developing 2D-material hollow fiber membrane for water desalination.Future work should be focused on further optimizing the structure of MXene laminar layer to improve the moderate desalination performance of the MXene hollow fiber membrane.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The work is supported by the National Natural Science Foundation of China (22038006,2192100621922805) and the Topnotch Academic Programs Project of Jiangsu Higher Education Institutions (TAPP).